LiFe 12.8v 4500 mAh over discharge cut off whats correct?

smoothisfast
smoothisfast Registered Users Posts: 16 ✭✭
I have a 12 Cell 12.8v 4500 LiFe battery pack that has a over discharge circuit cut off of < 8v

My LiFe solar charger has a over discharge cut off of <11.1v

Is the <11.1v too conservative?
Is there a way to calculate how much power in mAh is left in the battery in 11.1v?

Comments

  • mtdoc
    mtdoc Solar Expert Posts: 600 ✭✭
    Re: LiFe 12.8v 4500 mAh over discharge cut off whats correct?

    What do you mean by 12 cell 12.8V 4500?

    LiFePO4 cells have a nominal cell voltage of 3.0-3.3 V. Minimum discharge voltage is 2.8V - below that you damage the cell.


    Assuming you have a 4s3p pack (3 parallel string of 4 cells in series) - your nominal voltage would close to 12.8V and the pack should not be discharged below 11.2Volts
    (4x2.8V)

    If you have a BMS that has a Low voltage cut of 8 volts that is too low.

    Also - max charging voltage is 3.6 per cell so for this pack that would be 14.4 V

    Total Ah of your pack depends type of cell and how many in parallel.
    Is there a way to calculate how much power in mAh is left in the battery in 11.1v?

    No. There is very poor correlation of voltage with remaining capacity - esp with LiPO4.
  • smoothisfast
    smoothisfast Registered Users Posts: 16 ✭✭
    Re: LiFe 12.8v 4500 mAh over discharge cut off whats correct?
    mtdoc wrote: »
    What do you mean by 12 cell 12.8V 4500?

    Just wanted people to know there are 12 cells in the battery pack with 12.8v 4500 mAh
    mtdoc wrote: »
    Is there a way to calculate how much power in mAh is left in the battery in 11.1v?
    No. There is very poor correlation of voltage with remaining capacity - esp with LiPO4.


    Thats good to know. Are there simple gauges that can be wired to the battery to see remaining capacity?
  • mtdoc
    mtdoc Solar Expert Posts: 600 ✭✭
    Re: LiFe 12.8v 4500 mAh over discharge cut off whats correct?

    Thats good to know. Are there simple gauges that can be wired to the battery to see remaining capacity?

    There's the original Doc Wattson aka Watts Up meter for $60 HERE

    Or the $24 Chinese clone (has a backlight which is nice) HERE.

    A nice YouTube video review and comparison by Martin Lorton can be found HERE
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: LiFe 12.8v 4500 mAh over discharge cut off whats correct?

    Or a full blown Battery Monitor (Victron another good brand). Are they all programmable to support the Li-xyz chemistry--Not sure... You would need to research.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mtdoc
    mtdoc Solar Expert Posts: 600 ✭✭
    Re: LiFe 12.8v 4500 mAh over discharge cut off whats correct?

    A Victron or Trimetric or similar monitor with large external shunt is certainly a good way to track battery capacity - and no special programming needed to use with LiFePO4 - they simply track amps in and amps out as well as voltage. But they're not exactly inexpensive and probably overkill for the OPs small battery.

    I wonder what the application is.? Small LiFePO4s are most commonly used in remote control hobby applcations where the Watts Up type in line meters are very popular. Large LiFePO4 packs are used very commonly in Ebikes which is where my experience with them is from. I have both the Watts Up meter and the Doc Wattson. Both work well for this. But IMHO they would not be very good for a home sized solar other RE system application though. - The VIctron or Trimetric would be better choices for that....
  • RCinFLA
    RCinFLA Solar Expert Posts: 1,484 ✭✭✭✭
    Re: LiFe 12.8v 4500 mAh over discharge cut off whats correct?

    LIFePO4 batteries are 3.2v nominal during discharge. Absolute safety cutoff to prevent cell damage is 2.0vdc.

    For a 12.8 v battery there are four series cells. If your pack has 12 then there is three parallel strings or more likely each of the four cell voltage stack has three batteries strapped in parallel.

    The max charge cutoff is 3.6v per cell or 4x 3.6v = 14.4 vdc. Beware of lead acid battery chargers that may take them into equalize voltage of 15.5v or higher. It will stress the battery less if you can take 85% charge and kept them to 13.8v max. which is 3.45v per cell.

    There should be some cell balancing scheme to keep from overcharging or under discharging a single cell (3 parallel batteries in this case).
  • mtdoc
    mtdoc Solar Expert Posts: 600 ✭✭
    Re: LiFe 12.8v 4500 mAh over discharge cut off whats correct?
    RCinFLA wrote: »
    Absolute safety cutoff to prevent cell damage is 2.0vdc.

    That's way too low in my experience. I've destroyed cells in my ebike pack by letting them self discharge over a winter in my garage to higher voltages than that (wrecking my $500 battery pack.:cry:). BMS systems low voltage cut offs are much higher

    See THIS wikipedia entry and THIS from the endless-sphere wiki. Both cite 2.8V as the minimum discharge voltage
  • john p
    john p Solar Expert Posts: 814 ✭✭✭
    Re: LiFe 12.8v 4500 mAh over discharge cut off whats correct?

    2.0V is death and destruction in a very permanent way.. Li Fe cells hit a brick wall at 2.4 v and you need a good cut off switch to protect them from getting below 2.4v. Its much safer to have the switch operate at 2.6v.. better safe than sorry..
    I have destroyed cells large and small at 2.2 v.. BE WARNED.
  • RCinFLA
    RCinFLA Solar Expert Posts: 1,484 ✭✭✭✭
    Re: LiFe 12.8v 4500 mAh over discharge cut off whats correct?

    The problem comes into play when batteries are under load. If you are drawing greater then 5-10% C from the battery then 2.0 v is very appropriate. If you are escentually unloaded in off condition or only a few uA's of load then 2.0v is too low.

    The Rs (series equivalent resistance) goes up below about 3.0 vdc so the appearent cell terminal voltage will be lower by this Rs times the load current.

    Also, be careful about applying LiPo battery criteria to LiFePO4 batteries which have a lower cell voltage to begin with. The number for LiPo batteries is 2.4v to 2.8vdc. If you have a GSM phone which can hit peak draws of 2 amps from a 600 mAH battery with 3.75 vdc no load open cell voltage, it will drops to below 3.0 vdc with such a high load.

    For LiFePO4 the battery SOC is in the last 10% below about 3.0 vdc.
  • mtdoc
    mtdoc Solar Expert Posts: 600 ✭✭
    Re: LiFe 12.8v 4500 mAh over discharge cut off whats correct?

    Yes, LiPo (lithium polymer) chemistries are different than LiFePO4 - no argument there.

    But - load or no load, a LiFePO4 cell with a voltage of 2.0 is a dead cell - dead and gone.
  • smoothisfast
    smoothisfast Registered Users Posts: 16 ✭✭
    Re: LiFe 12.8v 4500 mAh over discharge cut off whats correct?
    mtdoc wrote: »
    I wonder what the application is.? Small LiFePO4s are most commonly used in remote control hobby applcations where the Watts Up type in line meters are very popular. Large LiFePO4 packs are used very commonly in Ebikes which is where my experience with them is from. I have both the Watts Up meter and the Doc Wattson. Both work well for this. But IMHO they would not be very good for a home sized solar other RE system application though. - The VIctron or Trimetric would be better choices for that....

    I'm using this to charge small electronic devices like phones, tablets, GPS ... more importantly I'm learning about solar power applications. Thank everyone for all the information thus far.
  • RandomJoe
    RandomJoe Solar Expert Posts: 472 ✭✭✭
    Re: LiFe 12.8v 4500 mAh over discharge cut off whats correct?

    Not to claim anyone is wrong, just wanted to mention my lone experience with over-discharging LiFePO4...

    I have a 4S1P pack of A123 LiFePO4 cells, 2.2AH. It has no BMS, it just has the pigtail hanging out to plug into a "smart" charger.

    A couple weeks ago I went braindead and left it connected on my bike (at the time powering a USB adapter to run my phone, and a WattsUp meter) for a week. When I found it the terminal voltage of the *pack* was 2V, 0.5V per cell! Yeah... Figured it was dead and gone, had to go to work, left it sitting on the desk.

    When I got home I found the terminal voltage had risen to 3.6-3.8V. What the heck - plugged it into the charger and told it to start. The charger showed the four cells were still balanced, said it was in "recovery mode" and very slowly brought the voltage back up. After a while it switched into normal charge mode, and eventually completed. Took a while!

    I'm sure the pack has lost some capacity, although I haven't tested it to find out just how much, but otherwise it is so far doing fine. Certainly surprised me, perhaps it's the quality of the cells? Sure cost me enough, so glad it's still working.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: LiFe 12.8v 4500 mAh over discharge cut off whats correct?

    Just make sure the power pack is somewhere "safe" that if it decided to melt down when unattended--It does not do it in your bedroom and start a house fire.

    I have no idea if it will fail or not--But I try to be very careful with Li type rechargeable batteries--They do store a lot of energy and can become very exciting if things go wrong. And I would not "trust" it for at least a few weeks "alone".

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mtdoc
    mtdoc Solar Expert Posts: 600 ✭✭
    Re: LiFe 12.8v 4500 mAh over discharge cut off whats correct?

    RandomJoe - thanks for the data point.

    A123 cells are supposed to be high quality - but you'll need to test the capacity of your pack under load to know what has really happened. I would be extremely surprised if there is not a problem. Did you put the pack together yourself?

    With my 16s 20AH 48Volt nominal ebike pack - once it was over discharged - I could charge it back up to 53V but it would sag quickly to below the bms LVCO with any signficant load. The capacity was shot due to damaged cells
  • RandomJoe
    RandomJoe Solar Expert Posts: 472 ✭✭✭
    Re: LiFe 12.8v 4500 mAh over discharge cut off whats correct?

    No, I bought the pack from BuddiPole which is a company that sells amateur radio gear. It's' just the four cells welded together, with a heavy-gauge pigtail for the power leads and the 5-wire harness for the balancing charger. All shrink-wrapped with some heavy shrink tubing.

    I haven't done a load-test yet, but it's been sitting for most of a week (used it on a couple rides after "killing" it, recharged last weekend, haven't ridden since) - just measured at 13.53V. For the most part I just use it to run my cell phone and if after dark lights on the bike. Pretty light load, even after a longer ride usually not even 1AH used so rarely did I get anywhere near the pack's capacity. Then I had a mental vapor-lock one day and forgot to disconnect it!

    This is the pack I have:
    http://www.buddipole.com/4sa1bapa.html

    The sad thing is if I run the pack through my Arduino lighting controller I programmed in a "kill switch" to shut everything down if the voltage was too low. I wasn't using the lights the day I left it connected so was only running the USB adapter! Ah well... :roll: